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Chapter 3
Timing Diagrams
3-4
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Handshaking I/O Timing Diagrams
This section compares of the handshaking I/O protocols and includes
timing diagrams for each:
•
Handshaking sequence for input operation
•
State machine for input operation
•
Timing specification for input operation
•
Handshaking sequence for output operation
•
State machine for output operation
•
Timing specification for output operation
Comparing the Different Handshaking Protocols
For an overview of all handshaking protocols supported by your
653
X
device, see Table 3-1.
Note
Whether an ACK or a REQ signal occurs first in the handshaking sequence depends
on the protocol and the direction of the transfer.
Table 3-1.
Handshaking Protocol Characteristics
Protocol
REQ/ACK
Polarity
Which REQ Edge
Requests Transfer
Where the
Programmable
Delay Is Located
Complementary Protocol(s)
Asynchronous Protocols
8255
Emulation
Active-low
Trailing
Between transfers
Long Pulse
Level ACK
Programmable
Leading
Before ACK
and between transfers
Level ACK
Leading-Edge
Programmable
Leading
Before ACK
and between transfers
Leading Edge
Long Pulse
Programmable
Leading
Pulse width and
between transfers
Long Pulse, 8255 Emulation,
and 8255
Trailing-Edge
Programmable
Trailing
Pulse width and
between transfers
Trailing-Edge